Break apart the heads of garlic and give the cloves a bash so the peel comes off easily, but leave them whole. Peel the onion and chop roughly. Put the garlic cloves and onion into a large saucepan and add 300ml of the milk. Bring up to bubbling, then lower the heat, cover the pan and simmer very gently for about 25 minutes, until the garlic and onion are really soft.
Stir in the sugar